Minor README edit, PPU register access optimization, and adding some …

…more cycles to blargg test

Still not even close to the full blargg test

README.md

@@ -77,3 +77,5 @@ This will create a `bin` folder in the repository and build the project from the
 - Explanation of mappers and bank switching: https://somethingnerdy.com/mappers-matter/
 - NES test ROM collection: https://github.com/christopherpow/nes-test-roms
 - NES input data and registers: https://stackoverflow.com/questions/66406934/whats-an-elegant-way-to-read-controller-input-for-nes-6502-assembly-on-ca65
+- PPU registers: https://www.nesdev.org/wiki/PPU_registers
+- List of NES mappers: https://www.nesdev.org/wiki/Mapper

src/core/ppu.cpp

@@ -37,10 +37,41 @@ void PPU::start() {
 
 void PPU::cycle() {
     if (cyclesOnLine) {
-        if (cyclesOnLine < 321) {
-            // TODO: Get nametable byte, attribute table byte, pattern table data as appropriate
-
-            // Output a pixel to the screen
+        // The first cycle is idle and should be ignored
+        if (cyclesOnLine < 257) {
+            /*
+                We perform four memory accesses, each one taking two cycles.
+                The PPU memory should not be accessed at this time, so we can just populate everything at once when needed.
+                The four accesses are:
+                - Nametable byte
+                - Attribute table byte
+                - Pattern table tile low
+                - Pattern table tile high
+            */
+        }
+        else if (cyclesOnLine < 321) {
+            /*
+                Now, we perform the following four memory accesses:
+                - Garbage nametable byte
+                - Garbage nametable byte
+                - Pattern table tile low
+                - Pattern table tile high
+                Each takes two cycles, and we repeat for each of the eight sprites.
+            */
+        }
+        else if (cyclesOnLine < 337) {
+            /*
+                We perform four memory accesses, each one taking two cycles, for two tiles.
+                - Nametable byte
+                - Attribute table byte
+                - Pattern table tile low
+                - Pattern table tile high
+            */
+        }
+        else {
+            /*
+                Two final nametable bytes are fetched. (Used for MMC5.)
+            */
         }
     }
 

src/cpu/cpu_test.cpp

@@ -96,7 +96,7 @@ void runBlarggCpuTest5Official() {
         std::this_thread::yield();
     }
 
-    for (int i = 0; i < 22000; i++) {
+    for (int i = 0; i < 22650; i++) {
         nes->cpu->cycle();
     }
 

src/memory/core_memory.cpp

@@ -37,7 +37,7 @@ addr_t CoreMemory::mapPPU(addr_t address) {
     if (0x2000 > address || address >= 0x4000) {
         throw std::runtime_error("Address is not in valid PPU register range!");
     }
-    return (address - 0x2000) % 8;
+    return address % 8;
 }
 
 /*